The optical system may be a lens system comprising one, a plurality, or a large number of lens elements in which each lens surface constitutes said transition, namely the transition between the lens material, for example glass or a synthetic material, and the medium surrounding the lens, for example air. The optical system may alternatively comprise components other than lens elements, such as a beam splitter, a birefringent plate and the like.
It is common practice to provide an optical coating, or cladding, such as an anti-reflection coating on the transitions so as to prevent unwanted reflections from occurring at said transitions. Not only do such reflections reduce the transmission through these transitions, but also the reflected radiation may reach unwanted positions in the relevant optical apparatus where it may cause troublesome effects.
An optical apparatus which has become very popular and is currently manufactured in large numbers is the playback apparatus for an optical record carrier in which an audio program is stored, which apparatus is known as CD player. This apparatus comprises, inter alia, an objective lens, preferably in the form of a single lens element having one or two aspherical surfaces with which a scanning beam from a diode laser is focused to form a scanning spot with a diameter of the order of 1 .mu.m on the information plane of the record. For novel uses of optical record carriers, for example as a storage medium for a digital audio program and for a video program or film, either or not in a digital form, the information contents of such a record carrier must be increased considerably, so that, with the same dimension of the record carrier, the information density must be increased considerably. This means that the information elements, for example in the form of pits in the information layer in which the information is stored in an encoded form, must be reduced considerably. To be able to read the smaller information elements separately, the scanning spot must also be reduced. The size of the scanning spot is proportional to .lambda./NA, in which .lambda. is the wavelength of the scanning beam and NA is the numerical aperture of the objective system. It has therefore been proposed to use a scanning beam having a smaller wavelength, for example 650 nm instead of the customary 860 or 780 nm and to increase the numerical aperture of the objective system.
The article "High-numerical-aperture lens systems for optical recording" in Optics Letters, vol. 18, no. 4, Feb. 15, 1993, pp. 305-307 describes how the numerical aperture can be increased considerably in a scanning device for optical record carriers, namely by providing a plano-convex lens between the customary objective lens and the record carrier, with the flat side of this lens facing the record carrier. This plano-convex lens is also referred to as SIL (solid immersion lens) in analogy with the immersion lens used in microscopes. An NA of 0.85 can be realized with the combination of the objective lens and the plano-convex lens.